In the field of packaging ICs, it is conventional to attach a “heat spreader” meaning a plate, with or without fins, to conduct heat generated during circuit operation away from the IC.
The effect of bonding a substrate and a chip with different coefficients of thermal expansion at elevated temperatures is that the organic substrates are warped, so that the finished module does not satisfy the flatness and/or coplanarity requirements that have been specified.
U.S. Pat. No. 6,166,434 illustrates a common approach, in which a chip is bonded to a rigid structure that makes thermal contact with the back side of the chip and is bonded to the substrate over the periphery of the structure. There is thus an extensive area over which stresses can be spread, which should alleviate warping.
A publication (EN886-0459, Research Disclosures July 1988) illustrates the use of a sheet of polyimide that flexes in response to stresses, so the IC is not in mechanical contact with a rigid substrate.
There still remains, however, a considerably problem of warping the substrate.